Among the known curing components for use in the production of polyurethane resins having useful mechanical properties, e.g. wear resistance, and chemical properties, polyisocyanates derived from non-yellowing diisocyanates, namely alkylene diisocyanates, cycloalkylene diisocyanates and aralkylene diisocyanates, excel in weather resistance and shelf life and particularly polyisocyanates having an isocyanurate ring, which is chemically stable, are known to be superior to the known biuret and adduct polyisocyanates in terms of weather resistance and shelf life.
Because of such desirable properties, those isocyanurate type polyisocyanates derived from alkylene diisocyanates, cycloalkylene diisocyanates, etc. are expected to be used in an ever-increasing spectrum of industrial uses. However, because of the high polarity of the isocyanurate ring, these polyisocyanates have been found to have the following disadvantages.
1) Although they are excellent in solubility, they must be used as dissolved in toxic polar solvents such as ethyl acetate, toluene, xylene, etc.
2) Despite the recent demand for high-solid coatings which has arisen from awareness of the risk of atmospheric pollution by volatile solvents, the coatings available today are still so high in viscosity that large amounts of solvents are required.
3) In respect of compatibility with other resins, the known polyisocyanates are not sufficiently compatible with fluorine-containing resins which are indicated in applications calling for high weather resistance. Thus, the isocyanurate type polyisocyanates so far proposed have met with considerable difficulties in commercial applications, because of the above problems associated with working environment, pollution and performance.
For overcoming these problems of the prior art technology, there has bene proposed a process which comprises using a diol containing 10 to 40 carbon atoms in the isocyanuration of such diisocyanates (Japanese laid-open Patent Application KOKAI No. 72013/1986) as well as a process which comprises using a polyester polyol containing 12-hydroxystearic acid as an essential component unit (Japanese laid-open Patent Application KOKAI No. 209124/1987), both processes aimed at improving the solubility of polyisocyanates in poor solvents of low polarity. However, these processes still have the following and other drawbacks and are, therefore, not fully satisfactory.
1) Though the polyisocyanates can be dissolved in poor solvents at room temperature or thereabouts, they are not soluble and produce white turbidity at low temperatures below 5.degree. C. so that they present problems in the winter months.
2) Because of their high viscosities, these polyisocyanates must be dissolved using large amounts of poor solvents and are, therefore, cannot meet the current demand for high-solid coatings.
3) Only compounds with low NCO contents sized so that a large proportion of the isocyanate component is required to economic disadvantage.